Drift mobility of holes in vacuum-deposited films of zinc tetraphenylporphyrin

Drift mobilities (μ) of holes in zinc tetraphenylporphyrin (ZnTPP) films were measured using the time-of-flight method. The plot of log μ vs. a square root of electric field (E12) fit a straight line and its extrapolation to E = 0 yielded an extremely small μ value of the order 10−10 cm2 V−1 s−1 at 20°C. The field dependencies of the hole mobilities at different temperatures were analyzed on the basis of the disorder formalism developed by Bässler and co-workers. As a result, the slow hole transport in the ZnTPP films was ascribed to a greater disorder parameter (σ = 0.15 eV) resulting from a nearly amorphous structure of the ZnTPP films.